Paulengr: Not sure where you are getting your cell counts from. Standard 18v LiIon battery packs use at least 5 3.6v batteries in series. (5 x 3.6V = 18V)
This is the case with both Ryobi's low profile 1.5Ahr & 2.0 Ahr batteries. The entire battery's capacity, as well as max & usable current, being limited by the specs of the cells themselves. The 2.0Ahr battery simply has higher capacity cells.
To get around that, the mid height 3.0 Ahr & 4Ahr batteries add a second bank of 5 cells in parallel. That both doubles the overall battery capacity (using the exact same 1.5-2.5Ahr cells) as well as their max & nominal current.
So when using a high current tool, like a circular saw, half inch drill with a large bit, lawn mower etc., you get more available current while also putting far less stress & strain on each cell, which helps to extend battery life as well.
The super tall 6Ahr & 9Ahr batteries add even more banks of 5 cells in parallel, giving yet more increases in capacity and high current draw performance. Not sure, but I believe the 9Ahr batteries use 4 banks of 5 cells with cells spec'ed at 2.25Ahr each @ 3.6V.
High Performance (HP) cells do things a bit differently, by adding additional circuitry & battery contacts that only really work with HP Brushless tools designed for them (with an extra set of contacts as well). This even allows a specially designed 2Ahr battery to be high performance, but only in an HP Brusless tool. If you use it in a non HP tool, not only are you not getting any extra performance, you may be limiting the performance of a high draw tool and damaging your pricier HP battery.
